Heretofore, a parking brake circuit as shown in FIG. 1 has been known. This prior art parking brake circuit is constructed such that, in order to introduce hydraulic operating fluid having a high pressure into a conduit 5, first and second main circuits 2, 3 are connected to first and second ports 1a, 1b of a hydraulically driven motor 1 for driving a vehicle, respectively, and further a counterbalance valve 4 is provided between the main circuits 2, 3, and still further one end of the conduit 5 is connected to a pressure receiving chamber 6a of an actuator 6 for actuating a parking brake. The parking brake circuit is adapted to release a parking brake 7 when hydraulic operating fluid is supplied into the pressure receiving chamber 6a of the parking brake actuating actuator 6 and, on the other hand, to operate or apply the parking brake 7 by the action of a compression spring and the like when the hydraulic operating fluid is discharged from the pressure receiving chamber 6a. In addition, in the parking brake circuit when the hydraulically driven motor 1 is driven by supplying hydraulic operating fluid through an operation valve not shown into either one of the first and the second main circuit 2, 3, the parking brake 7 is released. On the other hand, when the rotating operation of the hydraulically driven motor 1 is stopped by interrupting the supply of hydraulic operating fluid into the first and the second main circuits 2, 3, the parking brake 7 is applied or operated.
In the above described parking brake circuit, since the parking brake is released by the utilization of hydraulic operating fluid within either one of the first and the second main circuits 2, 3, which has become a high hydraulic pressure side. When either one of the first and the second main circuits 2, 3, on the side of high hydraulic pressure, becomes a low pressure (i.e., less than such a hydraulic pressure as releases the parking brake) because of a pumping operation of the hydraulically driven motor 1 caused, for example, by the vehicle travelling on a downward slope, the parking brake 7 is rendered operative undesirably so as to stop the rotating operation of the hydraulically driven motor 1.